The present invention relates to magnetic random access memory, and more specifically, to a spin-torque based memory device using a non-linear resistor to modulate read and write current paths.
A conventional spin-torque-based magnetic random access memory (MRAM) is based on a two-terminal, spin-torque-based magnetic memory element. This memory element uses the same terminals to sense the device resistance (at low voltage) and to switch the device state (at higher voltage). The sense, or “read” voltage operating point must be substantially smaller than the “write” voltage operating point to avoid read-induced device switching. For fast device switching, it is desirable to pass relatively large current through the device during the write operation, yet maintain the voltage across the device at a level substantially smaller than the breakdown voltage of the device. Such considerations lead to the desire for small device resistance to optimize the write operation. During the read operation, however, signal-to-noise considerations demand that the device resistance be substantially larger than the resistance of the series pass transistor. There is a conflicting requirement for low device resistance during the write operation and higher device resistance during the read operation. Because conventional magnetic tunnel junction (MTJ) spin-torque devices operate with similar read and write device resistance values, the overall circuit performance may be compromised, to provide adequate operating margins for reading and writing.
An alternative spin-torque-based MRAM element uses a three terminal geometry to completely separate the read and write circuit. Such devices, while allowing separate optimization of read and write parameters, are larger and more complex, adding to the cost of the circuit implementation.
It is therefore desirable to have a spin-torque based device that could combine the advantages of these two types of structures, while mitigate the shortcomings of them.